CLN: Removed dead GroupBy code

[WillAyd]

• closes CLN: Remove unused code in groupby.py #20456
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #20457 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #20457      +/-   ##
==========================================
+ Coverage    91.8%   91.82%   +0.01%     
==========================================
  Files         152      152              
  Lines       49223    49206      -17     
==========================================
- Hits        45191    45182       -9     
+ Misses       4032     4024       -8

Flag	Coverage Δ
#multiple	90.2% <ø> (+0.01%)	⬆️
#single	41.84% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/groupby.py	92.55% <ø> (+0.41%)	⬆️
pandas/util/testing.py	83.91% <0%> (-0.21%)	⬇️

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[jreback]

thanks!